For the purpose of reforming a gas to be reformed such as a top gas containing CO.sub.2 and H.sub.2 O, discharged from a reduction furnace such as a blast furnace and a shaft furnace for manufacturing reduced iron (hereinafter simply referred to as a "top gas"), into a reducing gas mainly comprising H.sub.2 and CO, the following three method are conventionally known:
(1) Method using a reforming tube filled with a catalyst
This method comprises reforming a gas to be reformed into a reducing gas, in a reforming tube filled with a catalyst, by means of external heat at a temperature of from 750.degree. to 850.degree. C. This method is widely adopted for reforming steam into a reducing gas rich in carbon monoxide (CO). As an example of this method, the Midrex method, developed by Midland-Ross Corporation of the United States, comprising reforming CO.sub.2 gas into a reducing gas rich in CO is known. This method is advantageous in that it allows continuous operations and permits easy maneuvering.
(2) Method using a regenerator type gas reforming furnace
This method comprises, by means of a gas reforming furnace of a heat-accumulating type, alternately repeating a heat-accumulating period and a heat-radiating period, i.e., a gas-reforming period, at certain time intervals, adjusting the atmosphere in the gas reforming furnace by switching over between the heat-accumulating period and the heat-radiating period, and causing a gas to be reformed to react with a hydrocarbon-containing gas in the gas reforming furnace at a temperature of from 1,200.degree. to 1,300.degree. C., thereby manufacturing a reducing gas. As an example of this method, the method based on the Kowper type methane decomposition furnace is known, which comprises manufacturing a reducing gas by causing steam to react with methane gas. There are also available the Purofer method, developed by Huttenwerk Oberhausen AG of West Germany, comprising manufacturing a reducing gas by reforming CO.sub.2 gas and the method comprising causing a top gas to react with a hydrocarbon-containing gas to manufacture a reducing gas and blowing the resultant reducing gas into a reduction furnace such as a blast furnace and a shaft furnace for manufacturing reduced iron, thereby using the top gas in recycle. This method is advantageous in that, because a gas to be reformed is reformed at a high temperature of at least 1,200.degree. C., the amount of produced soot, i.e., decomposed carbon, is slight and it is possible to utilize a higher hydrocarbon-containing gas.
(3) Method of partially oxidizing a gas to be reformed by means of oxygen
This method comprises oxidizing a part of a hydrocarbon-containing gas by means of pure oxygen to generate CO.sub.2 and H.sub.2 O, and causing the resultant CO.sub.2 and H.sub.2 O to react with the remaining hydrocarbon-containing gas, thereby manufacturing a reducing gas. A known example of this method is the Texaco method. This method is advantageous in that it requires only a very simple equipment and permits use of light oil and even heavy oil in place of a hydrocarbon-containing gas.
The methods (1) to (3) described above have however the following problems. In the method (1) mentioned above, higher hydrocarbon-containing gases other than methane gas cannot be used because of the production of a large quantity of soot, i.e., decomposed carbon. Use of a raw material gas containing large quantities of impurities such as sulfur leads to serious deterioration of the catalyst. Because of the restriction in strength of the material of the reforming tube, the attainable reforming temperature is from 700.degree. to 850.degree. C. at the maximum, and consequently, the temperature is somewhat insufficient to allow blowing of the resultant reducing gas into a blast furnace or a shaft furnace for manufacturing reduced iron. In the above-mentioned method (2), operations are rather complicated because of the need to open and close a plurality of valves at high temperatures for alternately switching over between a heat-accumulating period and a heat-radiating period, i.e., a gas-reforming period. In order to continuously manufacture a reducing gas, furthermore, it is necessary to install at least two reforming furnaces of a heat-accumulating type, thus resulting in higher installation costs. The above-mentioned method (3) is also problematic in that the use of pure oxygen requires high running costs and causes production of a large quantity of soot, i.e., decomposed carbon, during manufacture of reducing gas.
With a view to solving the problems involved in the above-mentioned methods (1) to (3), therefore, a method for manufacturing a reducing gas was proposed in Japanese Patent Provisional Publication No. 71,096/79 dated June 7, 1979 (corresponding to Japanese Patent Application No. 131,496/77), which comprises:
heating pebbles to be used in recycle to a prescribed temperature, in a pebble heating chamber, by combustion heat of a fuel gas; dividing said heated pebbles into two flows, one being introduced into a counter-flow type preheating chamber of gas to be reformed, and the other, into a parallel-flow type reforming chamber, both provided below said pebble heating chamber through dropping by their own weight; blowing a gas to be reformed containing CO.sub.2 and H.sub.2 O into said preheating chamber of gas to be reformed to preheat said gas to be reformed to a prescribed temperature through heat exchange with said heated pebbles in said preheating chamber of gas to be reformed, while, on the other hand, preheating a hydrocarbon-containing gas to a prescribed temperature by a preheating means; introducing said preheated gas to be reformed and said preheated hydrocarbon-containing gas into a mixing chamber for mixing; introducing the resultant gas mixture into said reforming chamber; reforming by reaction said gas mixture into a reducing gas rich in H.sub.2 and CO by heating said gas mixture through heat exchange with said heated pebbles in said reforming chamber; and utilizing in recycle said pebbles cooled to about 300.degree. C. through said heat exchange in said preheating chamber of gas to be reformed and said reforming chamber, by feeding said pebbles back into said pebble heating chamber by means of a transporting means such as a belt conveyor and an elevator (hereinafter referred to as the "prior invention").
The above-mentioned prior invention is described more in detail with reference to FIG. 1. FIG. 1 is a schematic descriptive drawing illustrating a reducing gas manufacturing equipment used for applying the method of the above-mentioned prior invention. As shown in FIG. 1, pebbles 34, having an average particle size within a prescribed range, to be used in recycle are fed into a pebble heating chamber 36 by an elevator 35, where pebbles 34 are heated to a temperature of about 1,500.degree. C. through heat exchange with combustion exhaust gases at a temperature of about 1,500.degree. C. produced as a result of combustion of a fuel gas, coming from a combustion chamber 46. The combustion exhaust gases cooled to a temperature of about 300.degree. C. through the heat exchange with the pebbles 34 are discharged to outside the system from the pebble heating chamber 36. The pebbles 34 heated to a temperature of about 1,500.degree. C. are divided into two flows and drop by their own weight into a counter-flow type preheating chamber of gas to be reformed 38 and a parallel-flow type reforming chamber 37, both provided below the pebble heating chamber 36. A gas to be reformed, containing CO.sub.2 and H.sub.2 O, such as a top gas, discharged from a reduction furnace such as a blast furnace and a shaft furnace for manufacturing reduced iron, is blown into the preheating chamber of gas to be reformed 38, where the gas to be reformed is preheated to a temperature of about 1,300.degree. C. through heat exchange with the heated pebbles 34 and then introduced into a mixing chamber 44. The pebbles 34, in the preheating chamber of gas to be reformed 38, cooled to a temperature of about 300.degree. C. through the heat exchange with the gas to be reformed are discharged onto a belt conveyor 40. On the other hand, a hydrocarbon-containing gas such as a natural gas is preheated to a temperature of about 600.degree. C. by a preheating means 45, and then introduced into the mixing chamber 44. The gas to be reformed preheated to a temperature of about 1,300.degree. C. and the hydrocarbon-containing gas preheated to a temperature of about 600.degree. C. are rapidly mixed in the mixing chamber 44 and are converted into a gas mixture. Rapid mixing almost prevents soot, i.e., decomposed carbon from being produced. The resultant gas mixture is introduced into the parallel-flow type reforming chamber 37, where the gas mixture is heated through heat exchange with the pebbles 34 heated to a temperature of about 1,500.degree. C. and is reformed by reaction into a high-temperature reducing gas at a temperature of about 1,200.degree. C. rich in H.sub.2 and CO, and the resultant reducing gas is taken out from the system. The pebbles 34, in the reforming chamber 37, cooled to a temperature of about 1,200.degree. C. through the heat exchange with the gas mixture drop by their own weight into a counter-flow type air preheating chamber 39 provided below the parallel-flow type reforming chamber 37. A part of the pebbles 34 in the reforming chamber 37 drop by their own weight also into the preheating chamber of gas to be reformed 38 to replenish the pebbles 34 in the preheating chamber of gas to be reformed 38. Air at the room-temperature is blown into the air preheating chamber 39, where the air is heated to a temperature of about 850.degree. C. through heat exchange with the pebbles 34 at a temperature of about 1,200.degree. C., and then introduced into the combustion chamber 46 as an oxygen source for combustion of a fuel gas in the combustion chamber 46. The pebbles 34, in the air preheating chamber 39, cooled to a temperature of about 300.degree. C. through the heat exchange with the air are discharged onto the belt conveyor 40.
The pebbles 34 discharged onto the belt conveyor 40 from the preheating chamber of gas to be reformed 38 and the air preheating chamber 39 are sieved through a screen 41 provided near the end of the belt conveyor 40 to remove pebbles reduced in size by abrasion and breakage, and the remaining pebbles having particles sizes within a prescribed range are fed back into the pebble heating chamber 36 by means of the elevator 35 for use in recycle. To make up the under-screen portion of the pebbles having passed through the screen 41, fresh pebbles 34 are fed from a pebble replenishing tank 42 from time to time into the elevator 35. The pebbles 34 flow in circulation through the above-mentioned reducing gas manufacturing equipment in this manner, and the flow rate of the pebbles 34 is adjusted by two wiper type scrubbing means 43 each provided at the pebble exit of the preheating chamber of gas to be reformed 38 and at the pebble exit of the air preheating chamber 39.
The above-mentioned prior invention has the following excellent advantages:
(1) Since it is not necessary to open and close valves at high temperatures, operation is easy and a high-temperature reducing gas can be continuously manufactured.
(2) The amount of soot, i.e., decomposed carbon produced during manufacture of a reducing gas is very small.
(3) Use of pebbles as a heat medium leads to a high thermal efficiency.
(4) The manufactured reducing gas, having a high temperature as about 1,200.degree. C., can be blown without any further treatment such as preheating into a reduction furnace such as a blast furnace and a shaft furnace for manufacturing reduced iron pellets.
The above-mentioned prior invention has in contrast the following problems. When light pebbles with a small particle size are used, for example in the counter-flow type preheating chamber of gas to be reformed 38, pebbles 34 cannot drop by their own weight but reversely flows upwardly under the effect of the pressure of the gas to be reformed flowing upwardly. In the prior invention, therefore, it is necessary to use heavy pebbles with a relatively large particle size. This requires installation of large-scale circulation facilities of pebbles such as a belt conveyor 40 and an elevator 35, resulting in disadvantages in the required space and the installation costs.